1. Field of the Invention
The invention relates to a controller for and a method of controlling a vehicle equipped with an internal combustion engine (hereinafter also referred to as “the engine”) and an automatic transmission connected to the engine, and more specifically, to a controller for and a method of controlling a vehicle in which the degree of opening of a throttle of the engine and the pressure of hydraulic fluid in the automatic transmission are controlled.
2. Description of the Related Art
In the field of engines mounted on vehicles, an electronic throttle system is available, in which an actuator for driving a throttle valve provided in each intake passage, which makes it possible to control the degree of opening of a throttle independently of the operation of the accelerator pedal by a driver. In the electronic throttle system, the throttle opening degree is controlled so that the optimal air intake amount (desired air intake amount) according to the operational conditions of the engine, such as the engine speed and the amount of depression of the accelerator pedal by a driver (accelerator pedal operation amount), is obtained. In such an electronic throttle system, the actual opening degree of the throttle valves is detected using the throttle opening degree sensors or the like, and the actuators for the throttle valves are feedback-controlled so that the actual throttle opening degree becomes equal to the throttle opening degree (desired throttle opening degree) at which the desired air intake amount is achieved.
Meanwhile, as a transmission for appropriately transmitting the torque and the rotation produced by an engine to the driving wheels depending on the drive conditions of a vehicle in the vehicle equipped with the engine, an automatic transmission is available, in which the speed ratio between the engine and the driving wheels is automatically and optimally set.
As automatic transmissions that are mounted on vehicles, a planetary gear-type transmission, in which a gear is set using clutches, brakes and a planetary gear(s), and a belt-type continuously variable transmission (CVT), in which the speed ratio is steplessly adjusted, are available.
In a vehicle equipped with a planetary gear-type automatic transmission, a shift map including the shift curves (gear change lines) used for establishing the optimal gear depending on the vehicle speed and the accelerator pedal operation amount is stored in an electronic control unit (ECU) or the like. In accordance with the shift map, the desired gear is calculated based on the vehicle speed and the accelerator pedal operation amount with reference to the shift map, and, based on the desired gear, the gear (speed) is automatically set by engaging or releasing each of clutches, brakes, and one-way clutches, which are frictionally engaging devices, into a predetermined state.
In general, a vehicle equipped with an automatic transmission is provided with a shift lever operated by a driver. The shift position of the automatic transmission is changed between the P position (parking position), the R position (reversing position), the N position (neutral position), the D position (drive position), etc. by operating the shift lever.
As a method of controlling such an automatic transmission, a technique for controlling the pressure of the hydraulic fluid in the automatic transmission based on the throttle opening degree of the engine is available. However, in the case where the hydraulic fluid pressure during a shift is controlled based on the throttle opening degree, if, in spite of the fact that the accelerator pedal operation amount is constant, the throttle opening degree varies due to the variation in the engine speed, for example, the hydraulic fluid pressure in the automatic transmission is varied, and the shift characteristics are also varied, which can cause a shift shock at the time of a shift.
As a method of suppressing such a shift shock, a method is available, in which the hydraulic fluid pressure in the automatic transmission is controlled according to the accelerator pedal operation amount when the automatic transmission is being shifted (see Japanese Patent Application Publication No. 2004-116760 (JP-A-2004-116760), for example). In addition, in such a control method, control for discharging (releasing) the hydraulic pressure in an engaging device of the automatic transmission is also performed when a release of the accelerator pedal is detected during a shift of the automatic transmission.
Meanwhile, in a process of controlling a vehicle equipped with an engine and an automatic transmission, control in which the throttle opening degree (the degree of opening of electronically-controlled throttle valves) is gradually reduced to gentle variation in the engine torque, which is sometimes called a dashpot control, is performed. Although it is possible to avoid a rapid variation in the engine torque by gradually reducing the throttle opening degree after a release of the accelerator pedal is detected during a shift, a certain amount of engine torque remains after the release of the accelerator pedal is detected, which can cause a shift shock. This will be described below.
In this kind of automatic transmission, as described above, when a release of the accelerator pedal is detected during a shift, in consideration of the ride quality and the like, instead of immediately changing the throttle opening degree to the degree corresponding to the fully closed state, which is a fully closed state in terms of throttle opening degree control, the throttle opening degree is gradually reduced to cause the engine torque to be gently reduced.
However, with regard to the hydraulic pressure for engagement of an engaging device of the automatic transmission, in the case of the conventional control method, the hydraulic pressure for engagement is discharged immediately after a release of the accelerator pedal is detected during a shift (see the hydraulic pressure command value shown in FIG. 7), which can cause a slip in the engaging device, such as the brake and the clutch, of the automatic transmission, which can in turn cause a shift shock. Specifically, although it is possible to avoid a rapid variation in the engine torque by gradually reducing the throttle opening degree after a release of the accelerator pedal is detected during a shift of the automatic transmission, a certain amount of engine torque remains after the release of the accelerator pedal. When the hydraulic pressure for engagement of an engaging device of the automatic transmission is discharged under conditions where a certain amount of engine toque remains as described above, the torque capacity of the engaging device (torque transmitted by the engagement) sufficient for the remaining engine torque is not kept in some cases. This can cause a slip in the engaging device, which can in turn cause a shift shock.